Meaning of the header in secondary structure of protein?

Question

[Bioinformatics313]

I download Secondary Structure Files from https://www.rcsb.org/pdb/static.do?p=download/http/index.html#ss. I want to know what does it mean the letter after PDB code in the secondary structure(header)?

>102L:A:secstr
  HHHHHHHHH  EEEEEE TTS EEEETTEEEESSS TTTHHHHHHHHHHTS  TTB  HHHHHHHHHHHHHHH
HHHHHH TTHHHHHHHS HHHHHHHHHHHHHHHHHHHHT HHHHHHHHTT HHHHHHHHHSSHHHHHSHHHHHHH
HHHHHHSSSGGG

I mean the letter 'A' in this case. Thanks.

[GenoMax]

Help for secondary structure files is on this page.

[Bioinformatics313]

Thanks for your reply. I read this link and many others, but I did not get the answer.

Answer 1

[Ram]

PDB identifiers usually consist of the 4-length alphanumeric identifier and the protein chain. I'm guessing 102L is the PDB identifier and A is the protein chain.

[Bioinformatics313]

Thanks I appreciate your answer . I understand it can be the chain of the protein, but I was confused because some the secondary structure in the file has different chain of protein with the same PDB id.

[Ram]

Yes, a protein can have multiple chains, and these chains can have different sequences of secondary structure elements.

[Bioinformatics313]

Thanks man. I really appreciate your answer .

[Ram]

No problem - if the answer resolved your question, can you accept it please?